The present invention relates to a transfer line or chain for a suspension of particles in constant quantity with a view to the conditioning thereof. The invention applies to the dosed transfer of all suspensions of particles and more particularly a suspension of particles with high specific masses.
The invention e.g. applies to the field of nuclear reprocessing, in connection with the transfer of suspensions of dissolving and shearing fines. The dissolving and shearing fines respectively correspond to the chemical elements of nuclear fuel and to the chemical elements of the material enveloping said fuel, which are insoluble in the solutions used for dissolving the fuel and its envelope during nuclear reprocessing. A solution used for this type of dissolving is e.g. nitric acid.
The dissolving and shearing fines constitute a highly radioactive waste material requiring conditioning. Thus, they have to be incorporated in constant quantities into a matrix, so as to provide good confinement guarantees. For these reasons, the suspended dissolving and shearing fines are transferred in constant quantities to appropriate conditioning means by a transfer line.
The invention also applies to the field of treating effluents, particularly for the transfer and sampling of saturated ion exchange resins, as well as to the chemical industry in connection with the dosing of e.g. solid additives.
FIG. 1 diagrammatically shows a known transfer line or chain making it possible to transfer a suspension of particles and in particular a suspension of dissolving and shearing fines in a constant quantity.
This transfer chain comprises a vessel 1, provided with stirring means such as a stirrer 3, a separating tank 5 located above vessel 1 and connected to means for forming a vacuum, such as a vacuum pump 6, a hydraulic guard 11 below the separating tank 5 and sampling means 15 between guard 11 and vessel 1.
The term lower part of an element constituting the transfer line means the lowest part of said element as opposed to the upper part which constitutes the highest part thereof.
A pipe 7 connects vessel 1 to separating tank 5. The upper end of pipe 7 penetrates the separating tank 5 by the lower part of the latter. An inclined plane 23 within the separating tank 5 is located just above the upper end of pipe 7. The gradient of inclined plane 23 decreases towards the centre of separating tank 5.
Pipe 7 is also connected to a duct 22 for blowing in air.
Pipe 7 connected to duct 22 and separating tank 5 connected to means for forming the vacuum constitute an air-lift system.
A pipe 13 connects the lower end of separating tank 5 to hydraulic guard 11, which is a generally cylindrical reservoir. The lower end of pipe 13 is immersed in hydraulic guard 11. A pipe 17 connects the outlet of said hydraulic guard 11 to sampling or removal means 15. A pipe 18 connected to the latter makes it possible to discharge the suspension removed by means 15 to conditioning devices. The unremoved suspension is discharged by a pipe 19 connecting the lower end of the removal means 15 to vessel 1.
The remainder of the description deals with the operation of the transfer line shown in FIG. 1. The suspension displacement direction is indicated by a solid line arrow S.
The suspension of particles is introduced into vessel 1, where it is homogenized by stirrer 3. This homogenized suspension rises by duct 7 up to the separating tank 5 connected to a vacuum pump 6. For this purpose, air is blown in at 21 into the lower part of pipe 7 by duct 22 which is connected thereto. The blowing in of air at 21 and the vacuum pump 6 make it possible to raise the suspension of particles into said pipe 7.
When the suspension of particles having air bubbles reaches the separating tank 5, the latter is projected against inclined plane 23 and drops towards the bottom of separating tank 5. Moreover, the air is evacuated to means for forming the vacuum, such as the vacuum pump 6 positioned above separating tank 5, which therefore makes it possible to separate the suspension from the air.
The air-free particle suspension is then discharged to the hydraulic guard 11 by pipe 13 connected to the lower end of the separating tank 5. The particle suspension arrives directly in the bottom of hydraulic guard 11 by said pipe 13 and is discharged by the upper end of guard 11 connected to pipe 17. This hydraulic guard, which has a constant suspension level makes it possible to prevent any sucking in of e.g. air or suspension particularly by pipes 17, 18, 19 positioned downstream of the hydraulic guard, whilst it also ensures that the suspension rises through pipe 7 under good conditions.
The particle suspension is transferred from hydraulic guard 11 to the removal means 15 by pipe 17. This removal means 15 makes it possible to remove a constant suspension quantity which is discharged by pipe 18 to appropriate conditioning devices. The unremoved suspension is returned to vessel 1 through pipe 19.
The particle suspension flow in the transfer line is ensured by the blowing in of air at 21 into duct 22 connected to the lower end of pipe 7 and by the vacuum pump 6 connected to separating tank 5.
FIG. 2 describes in greater detail the sampling or removal means 15 of a known transfer line or chain. This removal means 15 comprises a reservoir 25 within which is located a removal or sampling tank 27 and a wheel 29 with dosing buckets 33. Wheel 29 and dosing buckets 33 are indicated by dot-dash lines. The suspension displacement direction is indicated by the solid line arrow S.
Reservoir 25 is funnel-shaped and is connected by its lower end to pipe 19 discharging the particle suspension to vessel 1.
Removal or sampling tank 27 is also funnel-shaped and is provided at its lower end with a discharge orifice 28. Pipe 17 for supplying the particle suspension to the removal means 15 is laterally connected to the removal or sampling tank 27.
Wheel 29 comprises several inwardly turned dosing buckets 33 connected symmetrically with respect to a collector 31. This collector discharges the sampled suspension into the upper end of pipe 18. These dosing buckets are generally distributed in different planes. They are formed from generally metallic tubes. The shape of each bucket 33 is obtained by welding together several tubes, so that a non-continuous curvature is obtained. Wheel 29, preferably with a horizontal rotation axis, is a dosing bucket wheel, which is rotated by a not shown electric motor. Wheel 29 is partly located in removal or sampling tank 27 so that, during rotation, it removes a constant particle suspension quantity. The suspension level in tank 27 is kept constant by a permanent overflow of the suspension therefrom. Moreover, the orifice 28 made at the bottom of tank 27 permits the discharge of settled particles to the pipe 19 connected to reservoir 25. The suspension removed is discharged by collector 31 and then pipe 18 to appropriate conditioning devices.
In a known particle suspension transfer line, the particles with a high specific mass are deposited and form sediments in the different parts of said chain and particularly in the bottom of the hydraulic guard, in the removal or sampling tank and in the vicinity of welds on the dosing buckets. These deposits do not make it possible to remove suspension quantities representative of the initial particle concentration at the start of the transfer line. In the long run, they can also lead to the blocking of discharges from the transfer line and particularly from the removal or sampling tank.
These deposits increase with the mass, grain size and density of the suspended particles. This is the case with dissolving fines consisting of the following chemical elements: molybdenum, technetium, ruthenium, rhodium, palladium, uranium and plutonium, as well as shearing fines consisting of zircalloy or stainless steel. Thus, the chemical elements of the dissolving fines have a specific mass between 6 and 20 specific mass units and a grain size up to 1.5 .mu.m. The chemical elements of the shearing fines have a specific mass between 6 and 20 specific mass units and a grain size up to 100 .mu.m. Moreover, as the dissolving the shearing fines are radioactive, any residue of these fines in the transfer line constitutes a considerable disadvantage.